This article describes some generational differences and some interchangeable
parts between mk3 and mk4 cars in general. In the US market, the mk3 was sold as the jetta, passat, and passat wagon
only. The mk4 was
sold as the jetta, jetta wagon, passat, passat wagon, new beetle, golf, and
touareg SUV.

Aside from the obvious cosmetic differences in the body panels, suspension,
etc...

Comfort/equipment

The mk4 cars may have, depending on options/year, options that were not
available on mk3 cars such as: heated seats, trip
computers, 2 din radio, or in dash cd changer. If equipped with the cd
changer, your mk3 has it in the trunk. Mk4 had the cd changer in the trunk
but there was a Euro only in dash 6 cd changer. They also have different seats and
seat brackets (these are not interchangeable with earlier generation
cars). After 2004 and the introduction of the pumpe duse engine, mk4 cars
were slightly better equipped. Click here
for the cosmetic differences of the mk4 cars.

The mk4 jetta, jetta wagon, and golf all share the same front doors but are
different from the rear door back.

The mk3 cars used a vacuum driven door lock system to smoothly move the door
locks, the mk4 cars used an electric solenoid system which is more abrupt but
more reliable.

All mk4 cars after about 1999-2000 have an immobilizer in the key.
It's a chip inside the key which will shut off the engine after a few seconds if
it does not detect the correct chip. To see more specifics of the
immobilizer, see 1000q: immo FAQ.

Mk3 seat brackets are different from mk4 seats and are not interchangeable
without extensive modification and welding to the seat bracket. Most mk4 seats also have side airbags, no mk3 seats have these.

The mk3's tiny fixed center armrest was
replaced by a full sized tilting armrest that was much more durable.

The mk3 passat also did not have standard cupholders or a glovebox due to the airbag
location. The mk4 passat had both.

The mk4 cars also have convenience lighting at the bottom of the doors to
light the ground when the door is open.

Safety

Mk4 cars may have, depending on options/year, side and head curtain airbags,
stability and traction control. Mk3 cars
only had front airbags, none models have traction or stability
control.

Mk4 cars came with ABS standard in North America, mk3 cars had ABS as an
option. You may be wondering if it is
possible to retrofit ABS to a non-ABS equipped car -
anything can be done with enough time and money! However, it is so time
consuming and expensive that it would be easier to just buy another car that
already has ABS. You would have to change all the axles and brake rotors,
change the computers and wiring, splice an ABS pump and solenoids into the brake
hard lines, add a brake pedal sensor, and warning lights.

Some Mk4 had a full size spare tire, mk3 cars use a space saver spare.

Mechanical

Few mechanical accessories are interchangeable between mk3 and mk4 cars.
Some model
years alternator are interchangeable. All 1Z, AHU, ALH starter and starter solenoid
are the same in all VW TDI from 1996-2005.

Generally speaking, the maintenance on the mk4 cars is slightly more complicated
and will take slightly longer than on the mk3 cars.

The front suspension lower control arm bushings on the 1990-97 passat,
1985-2005 golf/jetta/newbeetle, 1995-2002 cabrio, 1990-1995 corrado are all the
same, you can replace them with Audi TT bushings for a firmer feel, all are
interchangeable. Note that there were 2 versions of the Audi TT bushings,
one small and one post-recall larger bushing.

Suspension and chassis are improved/stronger/safer on the mk4 cars. The
mk4 rear suspension no longer uses a strut on a torsion beam. The
mk4 rear suspension places the damper and spring on separate mounts on a torsion
beam.

All mk3 diesel cars have 4x100 wheel bolt patterns and all mk4 diesel cars
have 5x100 wheel bolt patterns except the passat, which uses 5x112. Refer
to the OEM VW/Audi
wheel gallery or 1000 answered questions: wheel fitment guide for more info on
wheels. Gasoline cars of each generation may use different bolt patterns/suspension than
the diesel cars.

All mk4 cars are pre wired for a trunk mounted cd changer. Mk3 jettas may or may
not be pre wired for a cd changer. No mk3 passat is factory pre wired
for a cd changer although it might have been done by the dealer.

Shifter linkage design and transmission are slightly different in mk3 vs. mk4
cars. They are not directly interchangeable but are very similar both
internally and externally. See 1000q:
shifter cable linkage adjustment for more details.

The engine oil filter in the mk3 cars is a spin-on metal cartridge type
accessed from below, mk4 cars use a replaceable soft filter in a housing type
accessed from above. MK3 cars require you to remove the plastic engine under
cover to access the oil filter. The
engine oil drain plug is easy to reach and does not require the removal of the
plastic undercover. This is a moot point because the long oil change
intervals require an oil filter change each time anyways. In mk4 cars, the engine oil filter is accessed
from above and is easy to reach, but the engine oil drain plug still requires you to remove the plastic engine under cover.
So you still need to remove the engine cover and drain the oil from below anyways.
Note: some people use an oil extractor on the mk4 cars so you don't have to
remove the under cover - this lets you suck the oil out fro the top.
However, it's recommended that you still drain the oil from below the car to
inspect the engine.

Engine

Note: the AHU/1Z engine uses the same mounting points as the older ME/MF/1V
engines used in the mk2 cars so the engines can physically fit into the mk2
engine bays. The mk1 cars need modification to the passenger side motor
mount to fit the engine.

The rods in all diesel mk3 mk4 cars that use the 1Z, AHU, and ALH engine, all
diesels up to and including 2003 for North American market, are the same, part #
028 198 401F .

The bore (79.9mm) and the stroke (95.5mm) are identical. In addition to the rods,
AHU ALH engines share the same pistons. All engines use 5 bearings for the crank.
All TDI pistons can physically fit into other generation cars.

The pistons in the mk3 jetta AHU and mk4 jetta, golf, new beetle ALH engine are the same. They are
slightly stronger than the 1Z pistons because of thicker ring lands but it does not make any significant
difference in stock or close to stock power levels. The piston ring on the newer pistons is closer to the top.
This reduces smoke
and improves emissions.

The engine oil pump on the mk3 cars is a roots type gear drive. The mk4 oil
pump is sprocket chain "gearotor" type driven off the crankshaft.
The advantage is less parasitic losses and faster cold oil pressure.

The mk3 oil pan is steel with a gasket, the mk4 oil pan is aluminum
with a liquid silicone gasket.

The mk4 aluminum oil pan is easily road damaged. This could lead to oil starvation and total
engine seizure and failure. The easiest solution is to put a metal engine
under cover, replacing the plastic under cover. The plastic under shield
is a dust, sound, and aero cover, but it should still always remain on unless
you replace it with an aftermarket shield because it still better protection
than no protection. It can also be damaged by improper towing with a wrecker, or wheel lift tow (the type of
tow truck that lifts only the front wheels with a metal yoke, as opposed to a
flat bed which pulls the entire car onto a flat bed).

The mk4 valve cover is magnesium.

The mk4 engine is mounted differently than the mk3 engine. The mk4
engine uses a "dogbone" pendulum mount in the rear. This helps
noise, vibration, harshness.

Water pump is driven off the accessory belt on the mk3 cars, water pump is
driven off the timing belt on the mk4 cars.

The turbocharger in the mk3 cars is a conventional turbo. The
turbocharger in mk4 cars are variable vane.

The intercooler is on the passenger side on the mk4, the intercooler is
on the driver's side on the mk3. Click here to read what
does an intercooler does and to see a picture, part of the article on
turbocharging.

The EGR (exhaust gas recirculation) valve on the mk3 cars is part of the
intake manifold and is not removable. The EGR valve on the mk4 cars is
attached with 3 bolts and is removable for cleaning or replacement.

The mk4 cars have an anti-shudder valve on the EGR valve body. It looks
like a throttle but it only closes at engine shut down, to cut off air.
All other times it should be open. The biggest advantage is that if you
have an engine runaway, turning off the key should cut off air to the engine and
should stop the engine runaway.

In the US, all mk3 TDI are direct injection. 1999.5-2003 mk4
are also direct injection but all 2004-2006 mk4 cars are pumpe
duse and got some other minor cosmetic differences.
See 1000q: direct injection vs. pumpe duse for
more details on this.
See 1000q: mk4 cosmetic changes to
see the year by year mk4 cosmetic differences.

All mk3 cars use a solid single mass flywheel, all mk4 cars use a dual mass
flywheel. A dual mass flywheel is made of 2 pieces
bound together with dampening material to help absorb driveline and engine
shocks. The significance of this is that the dual mass flywheels can
sometimes weaken and fail, damaging the engine and transmission. If you
ever do a clutch job on a mk4, an option is to replace the dual mass flywheel
with a solid flywheel. This will increase transmission
vibrations and increase overall noise and harshness.

Some Mk3 jetta and passat used a clutch assembly which is
stronger than the later years of the mk4 a4 clutch. The 1998-2000 mk4 diesels used a Luk clutch which is stronger than the
2000+ years that use a sachs
clutch which is only rated to 180 ft/lbs of clamping force.